1. Field of the Invention
This invention relates to a color proofing element and more particularly, it relates to a photoconductive layer coated with a releasable dielectric layer. This invention also relates to a process for making the inventive color proofing element.
2. Background of the Invention
Multicolor toner images produced by successive toner transfer from a photoconductor to a single receptor are well known in the art, both for powder toners with constituents intended to improve resolution on transfer and for use with magnetic brush development (U.S. Pat. No. 3,833,293). U.S. Pat. No. 3,612,677 discloses a machine designed to provide good registration when using successive color image transfer and U.S. Pat. No. 3,804,619 discloses special powder toners to overcome difficulties toners have in three color successive transfer.
U.S. Pat. No. 3,847,642 discloses a method and apparatus for transferring and fixing images formed by electrostatographic imaging techniques. A thin film of viscous material is applied to an imaging member prior to or after forming an electrostatic latent image thereon. The latent image is developed with electroscopic marking material and the resulting developed image is transferred by contact to a recording medium. A portion of the viscous material is also transferred to the recording medium with the image and is subsequently solidified, thereby fixing the image to the recording medium.
The production of multi-colored images by overlaying toned images on a photoconductor surface is also known. For example, U.S. Pat. No. 3,337,340 discloses liquid developers designed to minimize the "bleeding away of charge on the photoconductor surface" which occurs when recharging of an already toned surface is attempted. U.S. Pat. Nos. 4,155,862 and 4,157,219 disclose liquid toner formulations and apparatus for producing multicolor composite toned images on a photoconductor surface. U.S. Pat. No. 4,275,136 emphasizes the difficulties in ensuring that overlaid toner layers on a photoconductor adhere to one another. The addition of zinc or aluminum hydroxides coated on the colorant particles is used to solve the problem.
Many methods are used to aid the efficient transfer of toner from a photoconductor surface after toner development to a receptor sheet. U.S. Pat. No. 3,157,546 discloses overcoating a developed toner image while it is still on the photoconductor. A liquid layer having a concentration of about 5% of a film-forming material in a solvent is used at between 10 and 50 microns wet thickness. After drying, transfer is carried out to a receptor surface which has a mildly adhesive surface.
U.S. Pat. No. 4,066,802 discloses the transfer of a multi-toned image from a photoconductor, first to an adhesive carrier sheet and then to a receptor. The second stage involves the application of heat and pressure with a "polymeric or plasticizing sheet" between the image on the carrier sheet and the receptor surface. U.S. Pat. No. 4,064,285 also uses an intermediate carrier sheet which has a double coating on it comprising a silicone release layer underneath and a top layer which transfers to the final receptor with the multicolor image and fixes it under the influence of heat and pressure. U.S. Pat. No. 4,337,303 discloses methods of transferring a thick (high optical density) toned image from a photoconductor to a receptor. High resolution levels of the transferred images are claimed (200 l/mm). It is required to dry the liquid toned image and encapsulate the image in a layer coated on the receptor. Curing of the encapsulating layer is required with some formulations. The materials of this layer are chosen to have explicit physical properties which provide not only complete transfer of the thick toner image but also ensure encapsulation of it.
U.S. Pat. No. 4,477,548 teaches the use of a protective coating over toner images. The coating is placed on the final image and is not involved in any image transfer step. The coating may be a multifunctional acrylate, for example.
Transfer of certain types of composite multi-toned images is disclosed in the art. U.S. Pat. No. 3,140,175 discloses depositing microbeads containing a dye and a photoconductor on one electrode; exposing them through a colored original; and then applying a field between a first and second electrode thereby causing separation of charged and uncharged beads and transfer of the colored image to a receptor surface at the second electrode. U.S. Pat. No. 3,376,133 discloses laying down different colored toners sequentially on a photoconductor which is charged only once. The toners have the same charge as that on the photoconductor and replace the charge conducted away in image areas. However, it is disclosed that subsequent toners will not deposit over earlier ones. The final image of several toners is transferred to a receptor and fixed. U.S. Pat. No. 3,862,848 discloses normal sequential color separation toned images transferred to an intermediate receptor (which can be a roller) by "contact and directional electrostatic field" to give a composite multi-toned image. This composite image is then transferred to a final receptor sheet by contact and a directional electrostatic field.
U.S. Pat. No. 4,600,669 describes an electrophotographic proofing element and process in which successive liquid toned color images are formed on a photoconductive support. The composite image is then transferred to a receptor layer. The photoconductive layer has a releasable dielectric support coated thereon which may comprise a polymeric overcoat on the photoconductive layer which is transferred with the composite image. See also U.S. Pat. No. 4,725,867.
U.S. Pat. No. 4,515,882 describes an electrophotographic imaging system using a member comprising at least one photoconductive layer and an overcoating layer comprising a film forming continuous phase of charge transport molecules and charge injection enabling particles.
U.S. Pat. No. 4,565,760 describes a photoresponsive imaging member comprising a photoconductor layer and, as a release protective coating over at least one surface, a dispersion of colloidal silica and a hydroxylated silsesquioxane in alcohol medium.
U.S. Pat. No. 4,721,663 describes an improved enhancement layer used in electrophotographic devices between a top protective layer and the photoconductor layer.
U.S. Pat. No. 4,752,549 describes an electrophotographic receptor having a protective layer composed of a thermosetting silicone resin and a polyvinyl acetate resin.
U.S. Pat. No. 4,510,223 describes a multicolor electrophotographic imaging process. A general description of transfer of the toned image to an adhesive receptor is disclosed at column 15, lines 21-40.
As is known in the electrophotographic arts, toners are used to develop a latent electrostatic image produced on a photoconductive surface.
EPA Publication No. 498,535 A 1 (foreign counterpart to Assignee's copending U.S. application Ser. No. 07/652,572, filed Feb. 8, 1991) discloses a liquid electrophotographic toner having a coordinated association of steric stabilizer and charge directing moiety having a monovalent alkali metal or ammonium cation bonded thereto. The toner employs pigment particles.
U.S. Pat. No. 5,066,559 discloses liquid toners for developing electrophotographic images containing dispersed toner particles which are based on a polymer with multiple characteristics. These particles contain a thermoplastic resinous core with a glass transition temperature (T.sub.g) below room temperature which is chemically anchored to an amphiphatic copolymer steric stabilizer containing covalently attached groups of organic acid-containing moieties having a pKa less than 4.5 which in turn are chemically bonded to metal soap-containing compounds derived from organic acids having a pKa greater than 4.5. The toner particles so formed have advantageous properties of high charge/mass and good charge and dispersion stability.